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In this paper, we prove a general hardness amplification scheme for optimization problems based on the technique of direct products. We say that an optimization problem Π is direct product feasible if it is possible to efficiently aggregate any k instances of Π and form one large instance of Π such that given an optimal feasible solution to the larger instance, we can efficiently find optimal feasible solutions to all the k smaller instances. Given a direct product feasible optimization problem Π, our hardness amplification theorem may be informally stated as follows: If there is a distribution D over instances of Π of size n such that every randomized algorithm running in time t(n) fails to solve Π on 1/α(n) fraction of inputs sampled from D, then, assuming some relationships on α(n) and t(n), there is a distribution D' over instances of Π of size O(n⋅α(n)) such that every randomized algorithm running in time t(n)/poly(α(n)) fails to solve Π on 99/100 fraction of inputs sampled from D'. As a consequence of the above theorem, we show hardness amplification of problems in various classes such as NP-hard problems like Max-Clique, Knapsack, and Max-SAT, problems in P such as Longest Common Subsequence, Edit Distance, Matrix Multiplication, and even problems in TFNP such as Factoring and computing Nash equilibrium.
@InProceedings{goldenberg_et_al:LIPIcs.ITCS.2020.1,
author = {Goldenberg, Elazar and Karthik C. S.},
title = {{Hardness Amplification of Optimization Problems}},
booktitle = {11th Innovations in Theoretical Computer Science Conference (ITCS 2020)},
pages = {1:1--1:13},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-134-4},
ISSN = {1868-8969},
year = {2020},
volume = {151},
editor = {Vidick, Thomas},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITCS.2020.1},
URN = {urn:nbn:de:0030-drops-116863},
doi = {10.4230/LIPIcs.ITCS.2020.1},
annote = {Keywords: hardness amplification, average case complexity, direct product, optimization problems, fine-grained complexity, TFNP}
}